Measurement of a test substance such as a protein in a sample is currently carried out mainly by an immunoassay. As the immunoassay, various methods are known and practically applied, any of which methods utilizes a specific antibody against the test substance. Although creation of the specific antibody against the test substance can be carried out by a conventional method, it is laborious, so that the specific antibody is expensive.
On the other hand, aptamers which specifically bind to an arbitrary molecules are known. An aptamer which specifically binds to a desired target molecule can be created by a method called SELEX (Systematic Evolution of Ligands by EXponential Enrichment) (Non-patent Literature 1). In this method, the target molecule is immobilized on a carrier, to which a nucleic acid library comprising nucleic acids having vast kinds of random base sequences is added, and nucleic acids which bind to the target molecule are collected, which nucleic acids are then amplified by PCR, followed by addition of the amplified nucleic acids again to the carrier on which the target molecule is immobilized. By repeating this process about 10 times, aptamers having high binding abilities to the target molecule are concentrated, and the sequences thereof are determined to obtain aptamers recognizing the target molecule. The above nucleic acid library can be easily prepared by binding nucleotides randomly by an automated chemical synthesizer for nucleic acids. Thus, by the method using a library of nucleic acids having random base sequences, and which positively utilizes contingency, an aptamer which specifically bind to an arbitrary target substance can be created. An aptamer usually has a single-stranded region, and creation of an aptamer by a modified SELEX method is also known, wherein a complementary oligonucleotide can hybridize with the single-stranded region when the aptamer is not bound to the target substance, but cannot hybridize therewith when the aptamer is bound to the target substance (Non-patent Literature 2).
Patent Literature 1: JP 2003-294679 A
Patent Literature 2: JP 2003-294680 A
Patent Literature 3: JP 2003-294681 A
Non-patent Literature 1: Tuerk, C. and Gold L. (1990), Science, 249, 505-510
Non-patent Literature 2: Angew. Chem. Int. Ed. 2005, 44, 1061-1065
Non-patent Literature 3: Kazunori Ikebukuro et al., Nucleic Acids Research, 33(12), e108